This invention relates to a method of operating a fluidized bed reactor to decrease emissions of nitrous oxide (N.sub.2 O) while maintaining efficient sulfur capture in the reactor.
Fluidized bed combustion systems are well known and include a furnace section in which a primary, oxygen-containing gas, such as air, is passed through a bed of particulate material, including nitrogen-containing, carbonaceous fuel particles such as coal, sorbent particles, such as limestone, lime, or dolomite, for the capture of oxides of sulfur generated by the combustion of the coal, and solid products of combustion. The primary gas fluidizes the particulate material in the furnace section and promotes the combustion of the fuel particles at a relatively low temperature. These types of combustion systems are often used in steam generators in which a cooling fluid, such as water, is passed through a fluid flow circuit in a heat exchange relationship to the fluidized bed reactor to generate steam and permit high combustion efficiency and fuel flexibility, high sulfur adsorbtion and low nitrogen oxides (NO.sub.x) emissions.
A typical fluidized bed reactor utilized in the generation of steam is commonly referred to as a "bubbling" fluidized bed in which the fluidized particulate material forms a bed having a relatively high density and a well-defined, or discrete, upper surface. A more commonly used fluidized bed reactor is referred to as a "circulating" fluidized bed in which the fluidized particulate material forms a lower dense bed having a density below that of a typical bubbling fluidized bed and in which the primary gas has a fluidizing velocity which is equal to or greater than that of a bubbling bed. The primary gas passing through the lower dense bed entrains a substantial amount of fine particulate material to form an upper dispersed bed of particulate material, often to the extent that the primary gas is substantially saturated with the particulate material in the dispersed bed.
It is generally considered desirable to operate these circulating fluidized beds using relatively high internal and external solids recycling so that they are insensitive to fuel heat release patterns, thus minimizing temperature variations and stabilizing the sulfur emissions at a low level. The high external solids recycling is achieved by disposing a separator such as a cyclone separator at the furnace section outlet to receive the flue gases, and the particulate material entrained thereby, from the dispersed bed of the furnace section. The entrained particulate material is separated from the flue gases in the separator, and the cleaned flue gases are passed to a heat recovery section while the separated particulate material is recycled back to the furnace section. This recycling improves the efficiency of the separator, and the increased residence times of the fuel and sorbent particles result in more efficient use of the fuel and sorbent particles and, therefore, reduced consumption of the same.
Bubbling and circulating fluidized bed reactors also offer advantages in pollution control. For example, the emissions of NO.sub.x from fluidized bed reactors are relatively low compared to emissions from other conventional systems such as gas-fired systems and coal-fired power plants. Staged combustion in fluidized bed reactors permits even lower NO.sub.x emission levels to be achieved. Methods of operating a fluidized bed reactor using staged combustion to lower emissions of NO.sub.x are disclosed in U.S. Patent Nos. 4,308,810 and 4,773,339, both assigned to the assignee of the present invention, the disclosures of which are hereby incorporated by reference.
However, fluidized beds are not without problems. For example, there has been recent concern regarding the emissions of N.sub.2 O from fluidized bed reactors. It has been discovered that N.sub.2 O may act as an ozone layer scavenger, and N.sub.2 O is not readily broken down once released to the atmosphere. Currently, emissions of NO.sub.x and oxides of sulfur (SO.sub.x) are legislatively regulated and, in light of the adverse effects of N.sub.2 O on the ozone layer, it is likely that emissions of N.sub.2 O will also be regulated soon.
It has also been recently discovered that, although emissions of NO.sub.x by circulating fluidized bed reactors are relatively low compared to other conventional combustors, emissions of N.sub.2 O by circulating fluidized bed reactors can be significant. For example, N.sub.2 O emission levels from circulating fluidized bed reactors may typically be within the range of 50-200 ppm, whereas N.sub.2 O emission levels from boilers equipped with other devices may typically be within the range of 1-20 ppm. It is therefore important to reduce the emissions of N.sub.2 O from circulating fluidized bed reactors while simultaneously maintaining low emission levels for NO.sub.x and SO.sub.x.
Emissions of N.sub.2 O by bubbling fluidized beds is not thought to be as significant a problem as with circulating fluidized beds, nonetheless bubbling fluidized beds are falling into disfavor because of problems with lowering SO.sub.x emissions to acceptable values.